Agricultural fertilizers commonly include the plant macronutrients nitrogen and phosphorus. After fertilizer is applied to the soil of an agricultural field, these constituents are often prematurely depleted, which can have detrimental effects on the environment and significantly reduce the pool of available nutrients.
A principle cause of nitrogen loss is surface volatilization. This occurs proximate to the surface of the soil. Urea is a major nitrogen fertilizer. Urea nitrogen reacts with urease enzyme in the soil and breaks down to form ammonia gas. At or near the surface, there is typically little soil water to absorb these gases and, as a result, they escape into the atmosphere. This condition worsens when the urea forms of nitrogen are applied to the field but are not in direct contact with the soil, such as when urea is spread on corn residues or urea ammonium nitrate solution is sprayed on heavy residues of corn stalk or a cover crop. The rate of surface volatilization typically depends on the moisture level, temperature and surface pH of the soil. If the soil surface is moist, water in the soil evaporates into the air. Ammonia released by the urea is captured by the water vapor and lost into the atmosphere. Air temperatures greater than 50° F. and a soil pH greater than 6.5 significantly increase the rate of urea conversion to ammonia gases and resultant surface volatilization.
In certain applications, gaseous ammonia is applied to the soil of an agricultural field by metal application shanks that are introduced into the soil. If the soil is not thoroughly covered and packed behind the shanks, ammonia gas and its constituent nitrogen are lost from the soil surface before being absorbed into the soil water and converted to ammonium, which adsorbs to the soil particles.
Surface volatilization of nitrogen can also occur when ammonium forms of nitrogen (e.g., ammonium sulfate, di-ammonium phosphate, etc.) are applied to the surface of calcareous soils having a pH greater than 7.5. The reaction products formed when such ammonium fertilizers react with calcium carbonate tend to volatilize and dissipate into the atmosphere.
Another cause of nitrogen depletion from agricultural fertilizers is denitrification. This occurs when nitrate (NO3−) is present in the soil, but not enough oxygen is present to supply the needs of the bacteria and microorganisms in the soil. If oxygen levels are too low, such microorganisms strip the oxygen from the nitrate. This produces nitrogen gas (N2) or nitrous oxide (N2O), which volatilize readily from the soil. Denitrification increases when the soil is wet or compact or when excessively warm temperatures are encountered.
Leaching of nitrate is yet another cause of unwanted nitrogen loss. This occurs when the soil receives more incoming water (by either rain or irrigation) than it can hold against the force of gravity. As water migrates downward though the soil, nitrate-N, which is water soluble, moves with the water and is lost into the groundwater, from where it cannot travel against gravity back up into the soil profile. Although ammonium (NH4+) forms of nitrogen tend to leach very little in most soils, ammonium leaching can be significant in coarse-textured sands and some muck soils.
Both nitrogen and phosphorus can also be subject to premature depletion through runoff. Such runoff tends to occur when the soil receives more incoming water through rain or irrigation than the soil can accommodate. As water moves over the soil, some of the soil may be loosened and move with the water. The excess water can then carry the dislodged soil and any adsorbed fertilizer nitrogen and phosphorus away from the agricultural site. The offsite movement of such nitrogen and phosphorus due to runoff can be particularly severe in sloped or hilly terrains.
The depletion of nitrogen and phosphorus described above presents a number of problems and disadvantages. Because a significant portion of the plant-enhancing nutrients are lost, many agricultural fertilizer treatments tend to be inefficient and not optimally effective. A considerable amount of the active nitrogen and phosphorus nutrients applied to the field are wasted, plant growth may be slowed and/or an inferior crop may result. Applying additional fertilizers to make up for the nitrogen/phosphorus depletion can add considerable cost, both to the grower and to the consumer, can add to losses, and is not always effective. Another problem associated with depletion of nitrogen and phosphorus from agricultural fertilizers is the adverse environmental effects that frequently result. In particular, leaching of nitrates and urea as well as runoff of nitrogen and phosphorus-bearing sediments can contaminate and pollute nearby surface water (e.g., streams, rivers, lakes, ocean, etc.) and ground water (e.g., aquifers). Nitrate leaching is a significant environmental problem, because above certain levels, nitrate in drinking water is toxic to humans.
In addition, volatile nitrogen oxides, such as nitrous oxide (N2O), are known to be contributors to greenhouse gas (GHG), which can adversely affect the environment. Fertilizer runoff can cause phosphorus pollution of surface waters. When the amount of fertilizer applied to a site is increased to compensate for depletion, this only adds to the volume of potentially polluting crop nutrients introduced into the environment.